THEY DON'T WANT TO talk about it over at the bureau, but as you probably found out through an account in this newspaper the other day by staff writer Jane Seaberry, there's been a suggestion of naughtiness inside the FBI family. At least, Director Clarence M. Kelly thinks one of his agents has been naughty. After learning that agent Jack t. Colwell is allegedly living with a woman out of wedlock, Mr. Kelley sent him a personal letter saying that he was suspended because "you engaged in indescretions which reflect unfavorably on your moral character and which are not in keeping with the high standards expected of employees of the FBI. This conduct on your part is inexcusable and will not be tolerated by me."

The rest of the story has a certain quaint Victorian quality about it - proper young lady falls in love with dashing young man; disapproving parents pack her off on a long trip "to forget." Only this time, there's a bit of a switch: In another crisp letter, Mr.Colwell was told that he was being transferred to the FBI's New York office effective Nov. 1. By way of advice to the agent, Mr.Kelley wrote. "Henceforth you are to conduct yourself in such a manner as to be above reproach so that criticism of this nature will not again be necessary."

Mr. Colwell, in response, is suing the FBI, Mr. Kelley and three supervisors. He is seeking back pay, deletion of any mention of the dispute from his personnel record and permission to stay in the FBI's Washington field office. Nick F. Stames, the special agent in-charge of the field office here and a defendant in the suit, would say only that he didn't consider it a matter fit for public airing in this newspaper.

Well, unless the FBI can cite other better grounds for the action taken against Mr. Colwell - allegations that the agency may not have shared publicly yet - we think there is a public issue here. For one thing, according to the agent's attorney, the FBI so far has not alleged any specific sexual relationship, only that the Mr. Colwell was living in a house with a woman. In today's world, the mere sharing of shelter doesn't strike us as necessarily "criminal, infamous, dishonest, immoral or notoriously disgraceful conduct" or "conduct prejudicial to the government" - which are chief among the no-nos cited in the code of conduct for all Justice Department employees.

Perhaps Mr. Kelley was simply carried away by President Carter's remarks about couples "living in sin." But "sin" does not lend itself easily to definition in a set of government regs, nor should it. In any event, the code of conduct at least should be clarified to reflect a more modern view of what specific kinds of behavior really do impair a person's professional abilityor bring discredit to a government law-enforcement agency.